SAN FRANCISCO/NEW YORK (Reuters) – Stock investors are expected to tread carefully next week, as speculation about the timing of a U.S. Federal Reserve interest rate hike adds to concerns about valuations. Since hitting record levels two weeks ago, the U.S. stock market has struggled for direction, and investors said the next several sessions might prove no better. A run of better-than-expected economic figures failed to boost sentiment and instead renewed focus on whether the Fed will begin to raise short-term lending rates before long.